This type of pump rotor has been widely used in an internal gear pump such as lubricant oil pump, oil pump for automatic transmission or the like in a vehicle (for example, refer to Japanese Unexamined Patent Application, First Publication No. H11-343985). The internal gear pump includes an inner pump rotor having outer gear teeth; an outer pump rotor having inner gear teeth that mesh with the outer gear teeth; and a casing having a pumping-in port, through which fluid is pumped in, and a pumping-out port, through which the fluid is pumped out. The internal gear pump pumps in and pumps out the fluid by changing the cell volume formed between gear tooth surfaces of the rotors so as to carry the fluid when the rotors mesh with each other and rotate. In addition, the rotors mesh with each other and rotate while both end surfaces of the rotors in the direction of the rotation axes and an outer circumferential surface of the outer pump rotor slide on an inner surface of the casing.
Meanwhile, the internal gear pump, in general, is disposed between a fluid (for example, lubricant oil) supplier (for example, cylinder head) and an oil pan, which stores the fluid, and communicates with the oil pan through a strainer. When the internal gear pump is activated, the fluid in the oil pan is supplied to the inside of the internal gear pump from the strainer, and pumped in and pumped out by changing the cell volume in the internal gear pump, as described above, so as to be supplied to the cylinder head or the like.